The field of this invention relates to the liquid-phase oxidation of polyalkyl aromatics to their corresponding polycarboxylic acids. More particularly, the invention relates to the oxidation of tri, tetra, penta, and hexamethylbenzenes to their corresponding polycarboxylic acids in high yields.
We have not been able to achieve high yields of the polycarboxylic acid when polymethylbenzenes having three or more methyl substituents are oxidized to their corresponding polycarboxylic acids in the presence of a cobalt, manganese, and bromine catalyst utilizing a hydrocarbon solvent. This is particularly critical when polymethylbenzenes have four or more methyl substituents.
The reasons for this are not clear but the polycarboxylic acids produced when tri, tetra, penta, and hexamethylene are oxidized have a propensity to precipitate the cobalt and manganese from the reaction mixture. The cobalt, manganese, and bromine catalyst is also deactivated when the polycarboxylic acids have two carboxylic groups ortho to each other on the benzene ring. For durene, the deactivation of the catalyst occurs when the oxidation of durene to pyromellitic acid is about fifty percent complete. In polycarboxylic acids having more carboxylic acid groups ortho to each other, the catalyst deactivation occurs at conditions when the oxidation reaction is less than fifty percent complete. I have overcome these disadvantages by adding about 0.3 to 0.7 grams of water per gram of reaction mass, the preferred amount being about 0.4 to 0.6 grams of water per gram of reaction mass. The addition of the water takes place during the last third of the reaction, preferably just prior to completion of the reaction.
A process for oxidizing polyalkylaromatics having between 3 and 6 methyl groups on each benzene ring to their corresponding acids which comprises catalytically oxidizing the polyalkylaromatic feedstock with air in the presence of an aliphatic acid in an oxidation zone wherein liquid-phase conditions are maintained and wherein the weight ratio of aliphatic acid to the polyalkylaromatic is in the range of about 0.5-4:1.0 and the catalyst comprises one or more heavy metal oxidation catalysts and a source of bromine, the process comprises addition of a combination of sources of cobalt, manganese, and bromine components to provide about 0.05 to about 2.0 weight percent total metals on the polyalkylaromatic feedstock wherein there is present a weight ratio of bromine ions to total metal ions of about 0.5-8.0:1.0, a manganese content of about 10-50 percent by weight of the total metals, and the reaction is conducted at a temperature of about 100.degree. C. to about 275.degree. C. and about 0.3 to about 0.7 grams of water are added during the last third of the reaction per gram of total reactants. A process for oxidizing durene to pyromellitic acid which comprises catalytically oxidizing pyromellitic feedstock with air in the presence of an aliphatic acid in an oxidation zone wherein liquid-phase conditions are maintained and wherein the weight ratio of aliphatic acid to durene is in the range of about 0.5-4:1.0 and the catalyst comprises one or more heavy metal oxidation catalysts and a source of bromine, the process comprises addition of a combination of sources of cobalt, manganese, and bromine components to provide about 0.2 to about 0.4 weight percent total metals based on durene wherein there is present a weight ratio of bromine ions to total metal ions of about 0.5-3.0:1.0, a manganese content of about 10-50 percent by weight of the total metals, and the reaction is conducted at a temperature of about 100.degree. C. to about 275.degree. C. and about 0.3 to about 0.7 grams of water are added during the last third of the reaction per gram of the total reactants.
In the batchwise oxidation of the aromatic compounds having about 3 to 6 methyl units on each benzene ring the exothermic heat of reaction vaporizes some of the liquid solvent which is carried out of the reactor by the process air. The solvent is condensed and returned to the reactor as reflux. This liquid reflux is reheated toward the end of the reaction cycle to ensure temperatures high enough to bring the oxidation to completion. After reaction, the reactor contents are depressurized and the polycarboxylic acid is crystallized out to form a 50-60% solids slurry (close to the maximum solids concentration that is pumpable). The solids are filtered out and further processed into final product. My process is particularly suitable for the oxidation of alkyl aromatics having three or more methyl groups on each benzene ring to their corresponding polycarboxylic acids. The following hydrocarbons are particularly suitable as feedstock for my novel process: 1,2,4-trimethylbenzene, 1,2,3-trimethylbenzene, 1,2,3,4-tetramethylbenzene, 1,2,3,5-tetramethylbenzene, 1,2,3,4,5-pentamethylbenzene, and 1,2,3,4,5,6-hexamethylbenzene.
The oxidation of the alkyl aromatics such as durene, tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene is conducted batchwise. All of the hydrocarbon feedstock and most (90-99%) of the acetic acid and initial amount of catalyst components are charged at or near oxidation initiation temperature, preferably at about 100.degree. C. to about 165.degree. C., and at a pressure to maintain liquid-phase conditions. Then, pressurized air is injected into the reaction mixture and the reaction temperature is permitted to increase by heat evolved by the oxidation reaction to about 175.degree. C. to about 250.degree. C.
The total bromine added can be from a single source of bromine, for example, ionic bromine sources (HBr, NaBr, NH.sub.4 Br, and the like) or from a combined form of bromine, for example, organic bromines such as benzyl bromine, tetrabromoethane, and others.
My novel process relates to the liquid-phase oxidation of alkyl aromatics to polycarboxylic acids, particularly the oxidation of durene to pyromellitic acid using cobalt, manganese, plus bromine. A useful catalyst for my process is a cobalt-manganese-bromine catalyst and the oxidation is conducted at a temperature in the range of about 100.degree. C. to about 250.degree. C., which process comprises conducting a batch oxidation of durene so that the concentration of bromine in the first stage is 0 to about 0.5 mole per mole of metals while all the remaining bromine is added during the second stage. The total amount of bromine added is about 50 to about 800 weight percent of the total metal catalysts present, the reaction is completed in a non-continuous process at a temperature of about 140.degree. C. to about 250.degree. C.
The water about 0.4 to 0.6 grams per gram of reaction mass is added during the last third of the reaction. The preferred embodiment of my process comprises conducting a batch oxidation of the hydrocarbon so that in the first stage no bromine is added or the amount of bromine added is below 30 weight percent of the total bromine to be added. The reaction is completed in a non-continuous process at a temperature of about 120.degree. C. to about 250.degree. C. and during the last 5 to about 20 percent of the reaction time.
My novel process also relates to the liquid-phase oxidation of aromatic hydrocarbons having three or more alkyl groups attached to the benzene ring using cobalt, manganese, plus bromine. My novel invention is a process for the oxidation of tri, tetra, penta, and hexamethylbenzenes with molecular oxygen to benzene tetra, penta, of hexacarboxylic acid under liquid-phase conditions in the presence of a cobalt-manganese-bromine catalyst at a temperature in the range of about 100.degree. C. to about 250.degree. C.
The commercial process for the production of terephthalic acid, isophthalic acid, and trimellitic acid from p-xylene, m-xylene, and pseudocumene (1,2,4-trimethylbenzene), respectively, uses dioxygen as the oxidant, a soluble mixture of cobalt, manganese, and bromide as a catalyst, and acetic acid as the solvent. I have found that when other selected feedstocks are oxidized to aromatic acids, using this same process, that the aromatic acids severely inhibit and even prevent the reaction from occurring. This is because the aromatic acid precipitates the catalyst metals, cobalt, and manganese. For example, when durene is oxidized to pyromellitic acid, the pyromellitic acid concentration begins to increase near the end of the reaction and this acid precipitates the metals and prematurely stops the reaction before all of the durene is reacted. This results in low yields to pyromellitic acid. I have discovered a novel, unobvious method for preventing the precipitation of the metals. This novel procedure is the rapid addition of water near the end of the reaction. Water is a product of the oxidation of polyalkylbenzenes to aromatic acids, and the art of homogeneous oxidation teaches that its concentration should be minimized. It should be minimized because water is a catalyst deactivator. In this invention, we minimize the water concentration in the reactor until catalyst precipitation begins to occur. Then, I perform a process which is contrary to the teachings of the art--to add water rapidly to the reactor. I will now illustrate that: (1) catalyst metals are precipitated by certain aromatic acids in acetic acid, (2) that they become more soluble as the water concentration in the acetic acid increases, (3) that one can visually observe catalyst precipitation during a homogeneous oxidation, and that concomitant with catalyst precipitation, the reaction rate severely decreases, and (4) the rapid addition of water at the end of an oxidation of durene results in high yields of pyromellitic acid.
Table I illustrates that pyromellitic acid (1,2,4,5-tetracarboxybenzene, the product of durene oxidation) precipitates cobalt(II) acetate and manganese (II) acetate from a typical solution used in homogeneous oxidation. A typical water concentration inside a reactor during a homogeneous oxidation is about 20%. Example 21 on Table I illustrates that at this water concentration, using sodium bromide as the bromide source, 62% of the cobalt has precipitated and 93% of the manganese has precipitated. Similar results are illustrated for hemimellitic acid (the product of 1,2,3-trimethylbenzene) on Table II and for trimellitic acid (the product of pseudocumene) on Table III. Importantly, Tables I-III also illustrate that the catalyst precipitation ceases after a certain amount of water has been added to the acetic acid. For pyromellitic acid and hemimellitic acid this limit is about 40% water in the acetic acid, while for trimellitic acid the limit is about 20% (using hydrogen bromide as the bromide source--sodium bromide has higher limits).